Credit: Dinco422

Illustration of Concept for Titan Scenario Mission


With its 5150-km diameter, Titan is the largest moon of Saturn and the 2nd-largest planetary satellite in Milky Way after Ganymede (5276 km). But, it is the only natural satellite known to have a dense atmosphere with clear evidence of stable bodies of surface liquid. Its atmosphere is largely nitrogen with clouds of methane and ethane. The climate is dominated by seasonal weather patterns as on Earth with its rain and wind, which creates some dunes, rivers, lakes, seas and deltas.
Because its thick atmosphere, Titan’s pressure and density at the surface is greater than that on Earth, which it is at approximately 8 km altitude similar to our one.
The atmospheric temperature on Titan decreases fairly linearly from the surface up to approximately 60 km in altitude.
Outside the atmosphere, the solar intensity is only approximately 15 W/m2. And so, when coupled with the haze and clouds within the atmosphere that make the surface light very weak.
Because the relative humidity of methane on Titan is only ~50 percent, providing a thermodynamic out of balance, a body of pure methane cannot persist indefinitely on its surface.
We know that by the use of terrestrial empirical transfer coefficients who indicate us that the evaporation rate has been estimated at up to 1 m/yr. And, because of its strong link with the wind speed. The saturation vapour pressure of ethane is very low, as we see.

Titan Atmospheric Composition

Ethane suppresses the partial pressure of methane above mixed-composition seas, that has proved a complex Titan’s air-seas interactions. For now, we suppose that ethane probably migrates only over long periods, and so evaporation and precipitation of methane may be much more like terrestrial weather.
In fact, transient surface darkening has been observed at low latitudes on Titan in association with methane clouds, followed by brightening, suggesting that shallow flooding occurred, followed by evaporation.
The winds within the atmosphere blow fairly consistently in the same direction as the planetary rotation. At altitudes that can be considered for airship operation, below 20 km, the wind speed decreases from approximately 5 m/s to near 0 at the surface.

The gravitational acceleration on Titan (1.35 m/s2) is less than that of Earth’s moon. Although the atmosphere is composed mostly of nitrogen, the speed of sound within the atmosphere is about half that on Earth. This is mainly due to the low temperature of the atmosphere compared to Earth’s.
Density, temperature, and wind velocity of the atmosphere are critical in determining the feasibility of airship flight on Titan.


NASA at Saturn: Cassini's Grand Finale

Cassini captures sunlight glinting off of Titan's seas

Sea of Kraken Mare on Titan's moon


The Cassini’s radar discovers in 2006, when the Titan was in winter darkness, seas in the North Polar Region. And, after having more or less fully-mapped these seas in 2013, attention was drawn to exploration of liquid environments on Titan.

In order of ascending size, they are Punga Mare, Ligeia Mare, and Kraken Mare
These changes are particularly strong at the winter pole. Among the gases produced by photochemistry is ethane, which is a liquid at Titan conditions, and is expected to accumulate on the surface.
In Northern Winter Darkness position, Cassini radar observations in 2006, bodies of standing liquid were confirmed. Hundreds of radar-dark lakes, typically 20 km across, were discovered at about 70° N. Latitude.
As see in the picture below, the first sea to be observed was Ligeia Mare, a 300-400 km wide body, following by the smaller Punga Mare, who is closer to the North Pole, and the giant Kraken Mare sprawls over 1,000 km towards mid-latitudes. The three big seas were named by International Convention.
In the period of 2004-2010, Cassini were placed in the Southern hemisphere, where the place was better illuminated. It has discovered a body liquid of 70 by 250 km, later named Ontario Lacus. The Orbiter confirmed presence of ethane on it.
Further analysis of the near-IR data suggests that Ontario Lacus may in fact be muddy, and a bright margin is suggestive of a ‘bathtub ring’ of evaporate deposits.
Here, these are not solubility salts in terrestrial waters as Earth, but sure some organic analog has been probably deposited at the shrinking margins. In fact, some years later, a comparison between an optically measured outline and the margins in a radar image suggested that Ontario may have shrunk in extent due to seasonal evaporation and the very shallow regional slopes. So, Ontario lacus is most likely only a few meters deep.

Three seas have been mapped by Voyager & Cassini: Punga, Kraken, and Ligeia Mare, all in north polar region, all with ethane/methane/nitrogen compositions

So, we have liquid lakes and seas. Now, what is the temperature on it?
Attention was drawn to exploration of liquid environments on Titan after the discovery of seas in the North Polar Region by Cassini’s radar instrument in 2006 (the northern region was then in winter darkness) and the later mapping of these seas.
Liquid surface temperature is about 90-96K, which near freezing point of ethane and methane. Seas are ~96% liquid (ethane or methane) and 4% nitrogen (GN2). Finally, the density variations are about 30%.
We consider the preponderance of seas in the northern hemisphere as the result of the astronomical configuration of Titan’s seasons in the current epoch. That mean, the northern summer is less intense but longer than in the south. Because of that, we have a longer ‘rainy season’ in the north with big accumulation of that methane and ethane.
One of the most striking observations in the near-IR is of the Sun glinting off the surface of the lakes (picture below). In fact this, like the low radar reflectivity, told us that the roughness of the lakes must be exceptionally low, but it is a very iconic observation. The lake was appropriately named Jingpo Lacus named after the ‘Mirror Lake’ in China.

The glint of near-IR light from the mirror-smooth surface of Jingpo Lacus
The density of ethane is about 2/3 that of water, and the viscosity is rather similar, depending on temperature. Methane is a little less dense and rather less viscous. Many dissolved constituents (higher hydrocarbons, nitriles) may also be present and would increase the density, viscosity and dielectric constant. It is conceivable that compositional or thermal stratification may occur depending on how tides and wind-driven currents stir Kraken’s depths.
Titan have strong tidal forces, which give Saturn’s large gravity, but due to its 15.945 Earth day orbit period, change very slowly. And, because it pointing the same face toward its mother’s planet, its tidal bulge is varying only by ~9 percent over the day due to the eccentric orbit.
Nonetheless, tidal amplitudes of a few tens of centimeters have been calculated for Kraken, with current speeds of a few centimeters per second.
We consider that, the flatness of Titan’s seas posed a complex puzzle.
Why, in Titan’s low gravity and thick atmosphere, should the seas not have waves if the hydrocarbon liquids behave like water? No matter if it strong enough sometimes to form sand dunes, it is possible that winds are too light. Or, may be, the seas are enough viscous to damp waves.
NASA recognize that the question of wave height is important, not only for shoreline erosion effects but, in particular for the design of vehicles that might float on its surface.
Recent work found that the threshold wind speed for capillary wave generation should be ~0.4 m/s (1.3 ft/s) for methane-rich (low viscosity) seas, or ~0.6 m/s (2 ft/s) for ethane-rich seas (viscosity similar to water).
As we move towards northern summer solstice, Global Circulation Models (GCM) predict a rising probability that winds over Kraken or Ligeia may freshen enough to generate waves that are observable via the Sun-glint pattern on the sea surface or by its radar reflectivity. Once capillaries form, they can grow and become progressively larger gravity waves.
Given GCM predictions of maximum ~2 m/s (6.6 ft/s) in summer, the significant wave height is expected to reach ~80 cm (2.6 ft) or so, therefore shoreline erosion and beach processes are possible on Titan. Sediment transport, given the low-density contrast between ice bedrock and hydrocarbon liquid, and the low gravity should be readily mobilized in Titan’s seas.

The Huygens's Probe descent on Titan

Huygens's Probe beginning its descent through Titan's hazy cloud layers from an altitude of about 1,270 km. First, it had to decelerate from 18,000 to 1,400 km per hour with the following sequence of parachutes, which slowed it down to less than 300 km per hour. At an altitude of about 160 km, every thing were exposed to its atmosphere. When the Probe reached about 120 km, it replaced the main parachute by a smaller one to complete the 2.25 hour descent. 
At an altitude of 700 meters above the surface, the descent lamp was activated. This lamp was not to illuminate the landing site because the light levels on the surface of Titan are roughly 1,000 times less than sunlight and 1,000 times stronger than a full moon. Its purpose is to provide a monochromatic light source, which enable scientists to accurately determine the reflectivity of the surface. 
All in all, the surface mission lasted 1 hour and 10 minutes - considerably longer than had been anticipated, and no damage was done to the Huygens's Probe.
The Saturn’s distance from the Sun is 9.54 AU (1,427,000,000 km), which give the surface temperature of 94 K (–290 °F or –180 °C). This low temperature is also due to the greenhouse warming of methane which makes up a few per cent of the atmosphere, the rest being nitrogen. We know that, the 94 K is close to the triple point of methane who it can be like a greenhouse gas, just like water vapor on Earth. Similarly, methane forms clouds, hail and following by rain carves river valleys on Titan’s surface.
The weak sunlight that drives Titan’s hydrological cycle results in rain being rare, averaging only a few centimeters per year. These rains are probably expressed as massive downpours depositing tens of centimeters or even meters of rain in a few hours, but interspersed with centuries of drought.
Titan is tilted 26° on its spin axis providing a significant seasonal forcing climate. but because it takes 29.5 Earth years to go once around the Sun, its seasons are long. So, as well as its seasonal rainfall, the annual cycle also manifests in its stratospheric circulation, where wide swings in the abundance of various organic gases and hazes take place.

After THE GRAND FINALE, the Next Titan's Mission will MAY BE in 2038 with a Cryogenic Submarine in the Kraken Mare, methane-ethane Lakes discover by Cassini-Huygens Spacecraft 

Credit: Phase I Titan Submarine, July 2015, NASA
Titan is the largest moon of Saturn and the only natural satellite known to have a dense atmosphere.  And, since Cassini discovery, it was established that is the only object, other than Earth, with clear evidence of stable bodies of surface liquid.
The atmosphere is largely nitrogen with clouds of methane and ethane. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas and deltas, and is dominated by seasonal weather patterns as on Earth.
The submarine design faced many challenges. Pressures in depth sea-lakes in a liquid ethane is about 60 percent the density of water on Earth. Also, the smaller world of Titan give ~20% Earth’s gravity, which  meant that even at the maximum design depth of 1,000 m (3,281 ft) for the Cryogenic Sub provide a pressure of 1/10th  (10%) of that a terrestrial sub would encounter. That mean, the titan's sub would need to endure only ~10 bar of pressure at maximum depth, not as the 100 bar (10 MPa) pressure in Earth’s oceans.
Source: NASA/TM—2015-218831, July 2015 / Phase I Final Report: Titan Submarine 3 Steven R. Oleson / Glenn Research Center, Cleveland, Ohio / Ralph D. Lorenz / Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland / Michael V. Paul / The Pennsylvania State University, Applied Research Laboratory, State College, Pennsylvania

Credit: Titan Submarine: Exploring the Depths of Kraken Mare / 26th Space Cryogenic Workshop June 25, 2015
For NASA, important things about Submarine Design Approach must be driven by science. That is, the Traceability to Decadal Survey, the Astrobiology side about Evolution of hydrocarbons in universe, the Geology, who study atmosphere/sea exchange, surface, shore, waves and heat transfer.
To conceive the Submarine, it is necessary to have a High Level System for Operation in cryogenic sea (93K). Also, the Sub need to be Autonomous, with a Max Speed of 1 m/s and a Range of about 3000 km / 1 year. And, because the distance, a reliable DTE communication system.
Because the Submarine will be Submerged during 8 hours, the Surface Communications duration will be 16 hours. The possible depth in the sea must be up to 1 km, where the Pressurize sub reach ~150psi and 5-g axial loads, 2-g for lateral loads.
Many options exist to exploring the world of Titan. NASA focus on submarine because it provides more efficient, in-situ science system and, that is, give a very long range and maneuverability.
Briefly, Titan Submarine will be cylinder shapes with a pound of 1200 kg 6m long x 1.1m wide x 2m high, as: